Microcellular carbon foam and method

• US 5,208,003 A
• Filed: 10/13/1992
• Issued: 05/04/1993
• Est. Priority Date: 10/13/1992
• Status: Expired due to Fees

• Current Assignee: Martin Marietta Energy Systems, Inc.
• Original Assignee: Martin Marietta Energy Systems, Inc.
• Inventors: Brown, John D., Simandl, Ronald F.
• Primary Examiner(s): Foelak, Morton
• Application Number:
  US07/960,600
• Time in Patent Office: 203 Days
• Field of Search: 423/445, 423/449, 264/29.1, 264/29.7, 521/142, 521/149, 521/918
• US Class Current: 423/445.00R
• CPC Class Codes: C04B 38/0022 obtained by a chemical conversion or reaction other than those relating to the setting or hardening of cement-like material or to the formation of a sol or a gel, e.g. by carbonising or pyrolysing preformed cellular materials based on polymers, organo-metallic or organo-silicon precursors
• CPC Class Codes: C08J 2333/20 Homopolymers or copolymers of acrylonitrile C08J2355/02 takes precedence
• CPC Class Codes: C08J 9/28 by elimination of a liquid phase from a macromolecular composition or article, e.g. drying of coagulum
• CPC Class Codes: H01G 11/24 characterised by structural features of the materials making up or comprised in the electrodes, e.g. form, surface area or porosity; characterised by the structural features of powders or particles used therefor
• CPC Class Codes: H01G 11/32 Carbon-based
• CPC Class Codes: H01G 11/86 specially adapted for electrodes carbonisation or activation of carbon for the manufacture of electrodes H01G11/34
• CPC Class Codes: H01M 4/587 for inserting or intercalating light metals
• CPC Class Codes: Y02E 60/10 Energy storage using batteries
• CPC Class Codes: Y02E 60/13 Energy storage using capacitors
• CPC Class Codes: Y10S 521/918 Physical aftertreatment of a cellular product

Abstract

A microcellular carbon foam characterized by a density in the range of about 30 to 1000 mg/cm³, substantially uniform distribution of cell sizes of diameters less than 100 μm with a majority of the cells being of a diameter of less than about 10 μm, well interconnected strut morphology providing open porosity, and an expanded d(002) X-ray turbostatic spacing greater than 3.50 angstroms. The precursor for the carbon foam is prepared by the phase inversion of polyacrylonitrile in a solution consisting essentially of at least one alkali metal halide and a phase inversion solvent for the polyacrylonitrile.

Claims

1. A method for producing a microcellular carbon foam of a density in the range of about 30 to 1000 mg/cm³ and open porosity provided by well interconnected strut morphology, comprising the steps of:

a) dissolving polyacrylonitrile in a heated solution consisting essentially of at leas tone alkali metal halide and a solvent for the dissolution of the polyacrylonitrile in the heated solution and the phase inversion of the dissolved polyacrylonitrile to a liquid gel upon sufficient cooling of the heated solution;

b) placing the heated solution including the dissolved polyacrylonitrile in containment means provided with one of a nucleating promoting means and a relatively smooth surface;

c) sufficiently cooling the heated solution in the containment means to form a liquid gel of the polyacrylonitrile by phase inversion;

d) removing the solvent and substantially all of the dissolved alkali metal halide from the gel to provide a porous foam consisting essentially of polyacrylonitrile;

e) removing the residual solvent under vacuum;

f) cross linking the dry polyacrylonitrile foam at an elevated temperature;

g) curing the porous foam at an elevated temperature in an oxygen containing environment; and

h) heating the porous foam in an inert atmosphere to a temperature sufficient to carbonize the polyacrylonitrile forming the porous foam to provide the microcellular carbon foam.

2. A method for producing a microcellular carbon foam as claimed in claim 1, wherein the solvent is selected from the group consisting of propylene carbonate, tetramethylene sulfone, gamma-butyrolactone, and mixtures thereof.

3. A method for producing a microcellular carbon foam as claimed in claim 2, wherein the heated solution of solvent and at least one alkali metal halide containing the polyacrylonitrile consists essentially of about 75 to 30 wt. % polyacrylonitrile, about 68 to about 97 wt. % solvent, and a sufficient concentration of at least one alkali metal halide to promote solubilization and the unraveling of molecules of polyacrylonitrile in solution and maintain these molecules in extended positions to promote a firm gel, a interconnected strut formation and an expanded d(001) X-ray turbostatic spacing during the formation of the gel by phase inversion.

4. A method for producing a microcellular carbon foam as claimed in claim 3, wherein the concentration of at least one alkali metal halide is in the range about 0.05 to about 2 wt. %.

5. A method for producing a microcellular carbon foam as claimed in claim 4, wherein the alkali metal halide is selected from the group consisting of the bromides, chlorides, and iodides of lithium, potassium, and sodium.

6. A method for producing a microcellular carbon foam as claimed in claim 3, wherein the solution is heated to a temperature in the range of about 130°

to 160°

C.

7. A method for producing a microcellular carbon foam as claimed in claim 3, wherein the step of removing the solvent and substantially all of the at least one alkali metal halide from the gel is provided by exchange with a solvent therefore and the extraction of this last mentioned solvent with supercritical carbon dioxide.

8. A method for producing a carbon foam as claimed in claim 7, including the step of removing trace amounts of residual solvent from the extracted foam by vacuum sublimation.

9. A method for producing a microcellular carbon foam as claimed in claim 3, wherein the step of curing the porous foam at an elevated temperature is provided by heating the porous foam in air at a temperature less than about 180°

C. when residual alkali metal halide is present in the porous foam.

10. A method for producing a microcellular carbon foam as claimed in claim 3, wherein the step of curing the porous foam at an elevated temperature is provided by heating the porous foam in air at a temperature up to about 180°

C.

11. A method for producing a microcellular carbon foam as claimed in claim 3, wherein the step of heating the porous foam in an inert atmosphere to carbonize the polyacrylonitrile forming the porous foam is provided by heating the porous foam to a temperature of less than about 1200°

C. through a series of sequential heating stages at preselected heating rates with a soak period of a selected duration after selected heating stages.

12. A method for producing a microcellular carbon foam as claimed in claim 11, including the additional step of heating the carbonized microcellular foam in an inert atmosphere to a temperature in the range of about 2400°

to about 3000°

C. for converting the carbon foam to a foam containing a graphite phase.

13. A method for producing a microcellular carbon foam as claimed in claim 11, wherein the series of sequential heating stages is provided by first heating the of porous foam at a rate of about 2°

to 4°

C./min to a temperature of about 90°

C., a second heating at a rate of about 1°

C./min to a temperature of about 200°

C. followed by a soak period at 200°

C./min to a temperature of about 1°

C. to a temperature of about 250°

C. followed by a soak period at 250°

, a fourth heating at a rate of about 1°

C./min to a temperature of about 295°

C. followed by a soak period at 295°

C., a fifth heating at a rate of 1°

C./min to a temperature of about 500°

C. followed by a soak period at 500°

C., and a sixth heating at a rate of 2°

C./min at a temperature of 1100°

C. followed by a soak period at 1100°

C.

14. A method for producing a microcellular carbon foam as claimed in claim 13, wherein the soak period following the second, third and fourth heatings are for a duration of about 3 to about 5 hours, and wherein the soak period following the fifth and sixth heatings is for a duration of about 7 to about 9 hours.

15. A method for producing a microcellular carbon foam as claimed in claim 2, wherein the microcellular carbon foam is of a density in the range of about 30 to 400 mg/cm³ and has an expanded d(002) X-ray turbostatic spacing greater than 3.5 angstroms, wherein the polyacrylonitrile is in the solution in a concentration in the range of about 0.74 to about 7.8 wt. % (0.75 to 8.5 phs), wherein the solvent is in the solution in a concentration of about 89 to about 99 wt. %, wherein the containment means contains the nucleating promoting means for promoting nucleation and firmness of gels containing less than about 3 phs polyacrylonitrile during the formation thereof by phase inversion.

16. A method for producing a microcellular carbon foam as claimed in claim 15, wherein the solvent is propylene carbonate.

17. A method for producing a microcellular carbon foam as claimed in claim 2, wherein the containment means comprises a vessel, and wherein the nucleating promoting means is provided by a perforated metal liner or matted paper liner within the vessel.

18. A method for producing a microcellular carbon foam as claimed in claim 2, wherein the microcellular carbon foam is of a density in the range of about 400 to about 1000 mg/cm³ and has an expanded d(002) X-ray turbostatic spacing greater than 3.5 angstroms, wherein the polyacrylonitrile is in the solution in a concentration of about 7.8 to about 17 wt. % (8.5 to 20 phs), and wherein the solvent is provided by one of said mixtures and is in the solution in a concentration of about 78 to about 89 wt. %.

19. A method for producing a microcellular carbon foam as claimed in claim 18, wherein the mixtures of propylene carbonate and tetramethylene sulfone or propylene carbonate and gamma-butyrolactone comprise about 68 to 99 wt. % propylene carbonate and the balance at lest one of tetramethylene sulfone and gamma-butyrolactone.

20. A method for producing a microcellular carbon foam as claimed in claim 18, wherein the step of cooling the heated solution in the containment means to form the gel by phase inversion is achieved in a containment means provided with a relatively smooth surface.

21. A method for producing a microcellular carbon foam as claimed in claim 2, wherein the cross linking of dry polyacrylonitrile foam is achieved in the presence of maleic anhydride vapor at a temperature in the range of about 60°

to 100°

C.

22. A method for producing a microcellular carbon foam as claimed in claim 2, wherein a sufficient concentration of triethanolamine is added to the solution for inhibiting combustion of the dry foam containing the alkali metal when using drying and curing temperatures greater than about 180°

C.

23. A method for producing a microcellular carbon foam as claimed in claim 22, wherein the drying and curing steps are provided in air.